The present invention is directed generally to a class of organic semiconductor materials useful for the direct detection of ionizing radiation namely, π-conjugated molecules, and particularly π-conjugated polymers and polyaromatic hydrocarbons. The invention further relates to apparatus for detecting and measuring ionizing radiation.
Generally, semiconductor detectors of nuclear radiation operate by exploiting the fact that incident radiation, by interaction in the detector material, will create a charge pulse consisting of holes and electrons that can be separated under the influence of an electric field and the current detected by an external circuit.
Because they are readily available, radiation detector materials are generally semiconductor materials such as Si or Ge with lithium introduced into the semiconductor material so that it behaves as an extrinsic semiconductor. However, these materials have less than ideal band gap widths and high dark currents making them unattractive candidates for room temperature radiation detection devices. Further, the need to keep such Si(Li) and Ge detectors cooled to cryogenic temperatures poses significant limitations on the use of these materials in many applications, particularly where portability is desired.
Certain nonmetallic, crystalline solids such as mercuric iodide (Hgl2), lead iodide (Pbl2), thallium bromide (TlBr), indium iodide (Inl), thallium bromoiodide (TIBrI), mercuric bromoiodide (HgBrI), and cadmium zinc telluride (CdZnTe) are particularly useful as materials for room temperature radiation detection devices. However, for high radiation detection efficiency it is necessary to have materials that exhibit very low leakage currents (e.g., <10−7 A at electric fields of about 1000 V/cm to about 3000 V/cm) and thus, high electrical resistivity (i.e., 1/σ≧109 ohm·cm). One of the primary problems associated with nonmetallic, crystalline semiconducting materials lies in the presence of charge trapping defect sites and electrical instabilities caused by impurities in the starting material or introduced during subsequent processing. In addition, electrically active impurities may move under the influence of the applied field leading to unpredictable and variable electrical properties including high dark current and spectral distortions. The electrical resistivity of a material is a measure of its purity and defect concentration in the material and for most semiconducting material resistivities much greater than 108 ohm cm require semiconducting materials of the very highest purity and are thus, very difficult to achieve (M. Hage-Ali and P. Siffert, Semiconductors for Room Temperature Nuclear Detector Applications, 43, 245, 1995).
An alternative approach to radiation detection, particularly for the detection of neutrons, is the use of a scintillator material that emits visible light when exposed to radiation. The strong interaction of neutrons with hydrogen containing materials makes them particularly desirable as detector materials for neutron measurements. One method of imaging neutrons employs track detectors such as plastic scintillating fiber bundles externally coupled to photomultiplier tubes or photodiodes to record light and to determine the momentum of recoil protons generated within the scintillator. This approach has inherently high sensitivity due to the strong interaction with the protons of the plastic scintillator material and has been demonstrated for neutron energies down to about 14 MeV. However, charges generated by ionization of the plastic scintillator material must diffuse to specific sites that promote radiative recombination. Losses due to non-radiative recombination, isotropic emission of the scintillation light, attenuation and reflection at interfaces, and spectral mismatch with the photodetector can result in a signal that can be difficult to detect. Further, tracking recoil protons from typical fission neutrons (neutrons having energies between about 1 MeV to about 10 MeV) requires spatial resolution on the order of about 10 μm to about 100 μm, which is very difficult to achieve by this approach due to reduction of active volume and light output for reduced fiber diameter in the bundles (typical diameters are 50 cm to 500 μm).
Organic materials and particularly organic polymer materials and polyaromatic hydrocarbons are known in the art to possess very high resistance to the flow of direct current and thus should be desirable as a medium for the detection of ionizing radiation. Moreover, as discussed above, the interaction of ionizing radiation, neutrons, with the hydrogen of organic materials makes these materials particularly suitable as detector materials for detection of ionizing radiation, providing the organic materials possess a high enough electrical resistivity.